Articles | Volume 12, issue 3
Biogeosciences, 12, 653–679, 2015

Special issue: REgional Carbon Cycle Assessment and Processes (RECCAP)

Biogeosciences, 12, 653–679, 2015
Research article
02 Feb 2015
Research article | 02 Feb 2015

Recent trends and drivers of regional sources and sinks of carbon dioxide

S. Sitch1, P. Friedlingstein1, N. Gruber2, S. D. Jones3, G. Murray-Tortarolo1, A. Ahlström4, S. C. Doney5, H. Graven6, C. Heinze7,8,9, C. Huntingford10, S. Levis11, P. E. Levy12, M. Lomas13, B. Poulter14, N. Viovy15, S. Zaehle16, N. Zeng17, A. Arneth18, G. Bonan11, L. Bopp15, J. G. Canadell19, F. Chevallier15, P. Ciais15, R. Ellis10, M. Gloor20, P. Peylin15, S. L. Piao21, C. Le Quéré3, B. Smith4, Z. Zhu22,23, and R. Myneni24 S. Sitch et al.
  • 1University of Exeter, Exeter EX4 4QF, UK
  • 2Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
  • 3Tyndall Centre for Climate Change Research, University of East Anglia, Norwich NR4 7TJ, UK
  • 4Lund University, Department of Physical Geography and Ecosystem Science, Sölvegatan 12, 223 62 Lund, Sweden
  • 5Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA
  • 6Department of Physics and Grantham Institute for Climate Change, Imperial College London, London SW7 2AZ, UK
  • 7Geophysical Institute, University of Bergen, Bergen, Norway
  • 8Bjerknes Centre for Climate Research, Bergen, Norway
  • 9Uni Climate, Uni Research AS, Bergen, Norway
  • 10Centre for Ecology and Hydrology, Benson Lane, Wallingford OX10 8BB, UK
  • 11National Center for Atmospheric Research, Boulder, Colorado, USA
  • 12Centre for Ecology and Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK
  • 13Department of Animal {&} Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
  • 14Institute on Ecosystems and Department of Ecology, Montana State University, Bozeman, MT 59717, USA
  • 15Laboratoire des Sciences du Climat et de l'Environnement, CEA CNRS UVSQ, 91191 Gif-sur-Yvette, France
  • 16Biogeochemical Integration Department, Max Planck Institute for Biogeochemistry, P.O. Box 10 01 64, 07701 Jena, Germany
  • 17Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20740, USA
  • 18Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany
  • 19Global Carbon Project, CSIRO Oceans and Atmosphere Flagship, Canberra, Australia
  • 20University of Leeds, School of Geography, Woodhouse Lane, Leeds LS9 2JT, UK
  • 21College of Urban and Environmental Sciences, Peking University, Beijing 100871, China
  • 22State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China
  • 23Center for Applications of Spatial Information Technologies in Public Health, Beijing 100101, China
  • 24Department of Geography and Environment, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA

Abstract. The land and ocean absorb on average just over half of the anthropogenic emissions of carbon dioxide (CO2) every year. These CO2 "sinks" are modulated by climate change and variability. Here we use a suite of nine dynamic global vegetation models (DGVMs) and four ocean biogeochemical general circulation models (OBGCMs) to estimate trends driven by global and regional climate and atmospheric CO2 in land and oceanic CO2 exchanges with the atmosphere over the period 1990–2009, to attribute these trends to underlying processes in the models, and to quantify the uncertainty and level of inter-model agreement. The models were forced with reconstructed climate fields and observed global atmospheric CO2; land use and land cover changes are not included for the DGVMs. Over the period 1990–2009, the DGVMs simulate a mean global land carbon sink of −2.4 ± 0.7 Pg C yr−1 with a small significant trend of −0.06 ± 0.03 Pg C yr−2 (increasing sink). Over the more limited period 1990–2004, the ocean models simulate a mean ocean sink of −2.2 ± 0.2 Pg C yr−1 with a trend in the net C uptake that is indistinguishable from zero (−0.01 ± 0.02 Pg C yr−2). The two ocean models that extended the simulations until 2009 suggest a slightly stronger, but still small, trend of −0.02 ± 0.01 Pg C yr−2. Trends from land and ocean models compare favourably to the land greenness trends from remote sensing, atmospheric inversion results, and the residual land sink required to close the global carbon budget. Trends in the land sink are driven by increasing net primary production (NPP), whose statistically significant trend of 0.22 ± 0.08 Pg C yr−2 exceeds a significant trend in heterotrophic respiration of 0.16 ± 0.05 Pg C yr−2 – primarily as a consequence of widespread CO2 fertilisation of plant production. Most of the land-based trend in simulated net carbon uptake originates from natural ecosystems in the tropics (−0.04 ± 0.01 Pg C yr−2), with almost no trend over the northern land region, where recent warming and reduced rainfall offsets the positive impact of elevated atmospheric CO2 and changes in growing season length on carbon storage. The small uptake trend in the ocean models emerges because climate variability and change, and in particular increasing sea surface temperatures, tend to counter\-act the trend in ocean uptake driven by the increase in atmospheric CO2. Large uncertainty remains in the magnitude and sign of modelled carbon trends in several regions, as well as regarding the influence of land use and land cover changes on regional trends.

Final-revised paper